The Arms Monsters are last of the three of the 13 Demon Races wiped out by Fangires, whom they hate to be compared to and vowed revenge on the genocide of their races. Though they attacked/helped Otoya on a regular basis, the three eventually grew accustomed to his face when the King offers them a chance to be spared. Refusing to execute their friend, they were sealed away into small statues until Otoya saves them. However, upon making a Darkness Pledge to Otoya prior to his death, they are marked with orbs on their bodies and their original abilities sealed as they are then placed inside Castle Doran as its caretakers, watching over Wataru and the events of the outside world via tarot cards or playing chess or Hell's Old Maid to pass the time, the later game having a penalty on the loser. As a result of the pact, the Arm Monsters become Kiva's supporters, protecting the Wonderful Blue Sky Organization by allowing Kiva to access other powerful forms, possessing their fighting skills and mentality. They can sometimes get directly involved when needed to support Kiva. During the events of Cho Kamen Rider Den-O & Decade Neo Generations: The Onigashima Warship, the Arms Monsters were briefly possessed by Urataros, Ryutaros, and Kintaros in the 1930s.
Jiro is the human form of the violent and impulsive Garulu, the last surviving member of the Wolfen Race that were decimated by Rook in one of his Time Plays. Like those of his kind, Garulu has heightened senses and uses his claws to rip out human souls from their bodies before eating them. In the human guise of Jiro, Garulu sees the world as a place of utter corruption and usually prefers to be with Castle Doran as its lead caretaker. In 1986, Jiro is a regular at the Cafe mald'amour and attacks regulars in secret to feed on them. Recognizing Yuri as an "ideal wife" to revive the Wolfen Race through, Jiro joins the Fangire Hunters and becomes the first user of the Ixa System, saving Yuri from the Earwig Fangire and also alerting Otoya to his true nature, with the two at odds with each other unless Yuri's in any danger. But after Yuri reveal that her heart's for Otoya, Jiro left a lost man after exposing himself for what he truly was and attempts to get his revenge on Rook. But after the attempt, Jiro was forced to ask for Otoya's help, with them, Ramon, Riki, and Yuri defeating a weakened Rook together.
After becoming unable to kill Otoya to save himself (as per the King's offer), Jiro decides to leave town. However, Jiro comes back to unsuccessfully aid Otoya and Maya against Dark Kiva and gets sealed into the Garulu Saber for his efforts. He manages to escape imprisonment along with the other Arms Monsters because of Otoya and runs away after seeing Otoya and King fighting. In the fight's aftermath, before deciding to lead the Arm Monsters in keeping Castle Doran from running amok, Jiro questions Otoya's faith in them as he and the others promise to protect Wataru in his stead. This leads to Jiro now aiding Wataru in 2008, his Garulu Saber form used by Kiva to assume Garulu Form. Out of the Arms Monsters, Jiro supports his current condition as Wataru's ally out of respect for Otoya, even to the point of directly getting involved sometimes to honor his vows. In the The Queen of the Castle in the Demon World net spin-off movies, Jiro challenges Wataru to a card game when he accidentally ends up in Castle Doran, losing to him in Hell's Old Maid and suffers a humiliating punishment. He later tracks down Wataru to get back at him, finding Shizuka before she overwhelms his nose with an old fish and manhandles him for threatening Wataru.
Ramon is the human form of the cheerful Basshaa, the 127-year old survivor of the Merman Race. He has a cool yet childish attitude and is able to assume his true form by spinning then snapping his fingers. He can shoot pressurized water bubbles from his mouth and his pupils turn into crosshairs to increase his aim. Basshaa is Castle Doran's housekeeper, assuming the form of a 13-year-old boy in a School Gakuran outfit while using his cunning and telekinetic abilities to mess with his housemates in their games from time to time. In 1986, he lives in human society as a shoeshine boy while aiding Jiro out of common interest, though confused by the Wolfen's methods as he should be in hiding and feeding off of humans in secret. When the parlor he lived in closed, Ramon started to work at the Cafe mald'amour as a greeter until Jiro left and became a kakigoori seller when Jiro asked for his aid. After becoming unable to kill Otoya to save himself (as per the King's offer), Ramon decides to flee. But he is attacked by the King moments later and sealed into the Basshaa Magnum. This leads to Ramon now aiding Wataru in 2008, his Basshaa Magnum form used by Kiva to assume Basshaa Form.
Riki is the human form of the silent Dogga, the last surviving member of the Franken Race. He has unbeatable strength and feeds by grabbing a person and draining the soul out in the form of electrical discharges. He acts as Castle Doran's butler, assuming his true form by pounding his fists together. He can only speak in a slow, monotone voice, and is thus not as talkative as Jiro or Ramon, usually having one of them speak for him. In addition to his habit of popping his neck, Riki tends to eat things that are inedible to normal people. In 1986, he lives in human society as a masseur in Ramon's parlor before moving into the Cafe mald'amour to work there for a bit, falling in love with Yuri after eating her food. When Jiro left, Riki joined Ramon to became a kakigoori seller until Jiro asked for their aid. After becoming unable to kill Otoya to save himself (as per the King's offer), Riki decides to flee. After seeing Ramon sealed by the King, Riki attempts to run away, but is sealed into the Dogga Hammer. This leads to Riki now aiding Wataru in 2008, his Dogga Hammer used by Kiva to assume Dogga Form.